The present device relates generally to semiconductor devices and, more particularly, to semiconductor devices and their manufacture involving a proportional-to-absolute temperature (PTAT) current source in the device.
In recent years, the semiconductor industry has realized tremendous advances in technology that have permitted dramatic increases in circuit density and complexity, and equally dramatic decreases in power consumption and package sizes. Present semiconductor technology permits single-chip microprocessors with many millions of transistors, operating at speeds of hundreds of MIPS (millions of instructions per second) to be packaged in relatively small, semiconductor device packages.
A problem with packaging these high-density circuits in relatively small, semiconductor device packages is a significant increase in ambient temperatures due to the power dissipation associated with the density and operational speed of the circuits. In an effort to control this overall increase in temperature, many such semiconductor circuits are air-cooled and include alarms and shut-back circuits for temperatures increasing beyond specified operating conditions. Unfortunately, as the complexity of integrated circuits has increased, the demand for highly functional and reliable power sources for use in various applications has also increased, and the more complex circuits require internal temperature controls to ensure proper operation.
In an attempt to address this temperature issue, it is often necessary to provide a local reference voltage of a known value that remains stable with both temperature and process variations. A common solution is a bandgap reference circuit. A bandgap reference circuit provides stable, precise and continuous output reference voltages for use in various analog circuits. Recently, it has become necessary for many commercial integrated circuits to operate at less than the conventional five-volt power supply voltage, such as three volts. As a result, bandgap reference voltage circuits must operate over a power supply range from over five volts down to three volts and less. The output reference voltage provided by known bandgap reference circuits, however, typically varies somewhat with respect to one or more of factors, such as temperature and manufacturing processes. Moreover, some known bandgap reference circuits fail to function when the power supply voltage is lowered to three volts.
One method of providing a voltage reference is to provide a stable reference current through the base-emitter voltage Vbe of a forward-biased bipolar transistor, which provides a fairly linear function of absolute temperature T in degrees Kelvin. When using a bandgap reference, the reference voltage is obtained by compensating the base-emitter voltage of a bipolar transistor Vbe for its temperature dependence (which is inversely proportional to temperature) using a proportional-to-absolute temperature (PTAT) voltage. The difference between the base-emitter voltages Vbe1 and Vbe2 of two transistors, referred to as xcex94Vbe, that are operated at a constant ratio between their emitter-current densities forms the PTAT voltage. The emitter-current density is conventionally defined as the ratio of the collector current to the emitter size. Because the two silicon junctions are operated at different current densities (J1, J2), the differential voltage, xcex94Vbe, is a predictable, accurate and linear function of temperature.
Reference circuits of this type are present in many applications ranging from purely analog, mixed-mode, to purely digital circuits. The demand for low voltage references is especially apparent in mobile battery operated products, such as cellular phones, pagers, camera recorders, and laptops. Consequently, low voltage and low quiescent current flow are intrinsic and required characteristics conducive toward increased battery efficiency and longevity. Low voltage operation is also a consequence of process technology. This is because isolation barriers decrease as the component densities per unit area increase thereby exhibiting lower breakdown voltages. Unfortunately, due to increased demands in operating voltage levels, lower dynamic range will be required which, in turn, necessitates that reference voltages be more accurate.
In view of the above demands, PTAT circuits are often used to provide differential current to such semiconductor circuits. PTAT current sources are often limited, however, and can exhibit operational discrepancies at various states, such as during startup of an integrated circuit device. PTAT current sources are particularly limited in applications for oscillator circuits. In such situations, the presence of an operational amplifier that can include a negative feedback mode that can conflict with the function of the current source. In some implementations, the conflict can cause incorrect frequency at startup, which can seriously hinder the functionality of the integrated circuit device in which the current source is being used. PTAT sources can also exhibit ripple effects that distort or otherwise adversely affect the output of the source and the circuit being powered. These and other difficulties associated with PTAT current sources have been a hindrance to the advancement of the semiconductor industry.
A further issue is the financial constraint that these circuits be implemented using relatively simple processes, such as standard CMOS, bipolar, and BICMOS technologies. Thus, when attempting to address the above-mentioned problems, it is helpful to use circuitry that does not burden the semiconductor manufacturing processes.
The present invention improves the application of PTAT current sources to semiconductor devices including those applications addressed above. The present invention is exemplified in a number of implementations and applications, some of which are summarized below. Advantageously, the reliability and operability of semiconductor devices can be improved using a current source, according to the present invention, that exhibits stability during startup, operates at low voltages and can be applied in connection with operational amplifiers. An example embodiment of the present invention is directed to a PTAT current sourcing circuit that includes first and second current paths and a folded current-drawing arrangement in which first and second bipolar transistors provide a substantially-reduced minimum operating voltage. In a more particular embodiment, the PTAT current sourcing circuit comprises a first current path including a pair of cascoded MOS-type transistors inter-coupled at a first node, and including another MOS-type circuit in series with the pair of cascoded MOS-type transistors. A second current path, in parallel with the first current path, includes a pair of cascoded MOS-type transistors inter-coupled at a second node, and includes another MOS-type circuit in series with the pair of cascoded MOS-type transistors in the second current path. The first and second current paths are adapted to include feedback and to form a current loop.
Another aspect of the present invention is directed to a PTAT current source circuit, such as described above, and further includes a circuit for mirroring the current flowing in the main resistive path.
Yet another aspect of the present invention is directed to a PTAT current source circuit including a circuit having a bandgap reference voltage that is controlled by the feedback of the first and second current paths.
The above summary is not intended to describe each illustrated embodiment or every implementation of the present invention. For example, other aspects of the invention are directed to methods and systems employing one or more of the above features. The figures and detailed description that follow more particularly exemplify these embodiments.